1. Field of the Invention
The present invention relates to a modulating method for transmitting digital data through a communication channel and a demodulating method for recovering the transmitted digital data at a receiver.
2. Discussion of the Related Art
The demand for bandwidth on the radio frequency spectrum is dramatically increasing with emerging communication technologies, including personal communication networks, cellular telephones, and mobile computing. As these technologies mature, ever increasing amounts of data are transmitted using the limited spectral resources.
Successful digital transmission requires that the spectrum width of the transmitted signal be confined to the available bandwidth in the channel. Information theory has demonstrated that the rate at which digital information can be transmitted through a communication channel cannot exceed the channel capacity C, which is defined by the equation: EQU C=B1n(1+S/N),
where S/N is the signal-to-noise ratio, and B is the available bandwidth of the channel. Because the signal-to-noise ratio is limited by the physical characteristics of the channel and the communication equipment, improvement to the signal-to-noise ratio is mostly not achieved by improving the coding scheme. A coding scheme may, however, improve the utilization of the bandwidth B, and thereby increase the data rate achievable. Thus, in the prior art, a number of coding schemes have been devised to better utilize the available bandwidth.
A digital modulation method is a coding scheme which represents an information stream by symbols defined using the values of a known signal's amplitude, phase, frequency parameters, or a combination of these parameters. Many digital modulation methods modulate a sinusoidal signal. One example of a digital modulation method is the phase shift-key (PSK) modulation in which the phase of a carrier signal is modulated by a symbol to be transmitted. Similarly, in amplitude shift-key (ASK) modulation, the amplitude of a carrier is modulated by a symbol to be transmitted. In a quadrature-amplitude (QAM) modulation, both the amplitude and the phase of a carrier are modulated by a symbol to be transmitted. In frequency shift-key (FSK) modulation, both the frequency and the phase of a carrier are modulated by a symbol to be transmitted.
Each modulation scheme used in the prior art falls substantially into one of the following three categories: (i) quadrature amplitude and phase modulation, (ii) m-ary orthogonal signaling, (iii) simultaneous transmission of orthogonal sinusoids.
A simple conventional phase modulation scheme is the binary PSK modulation, or BPSK modulation. In BPSK modulation, each symbol represents one of the two values of a single bit (i.e. `0` or `1`). In BPSK, each symbol is encoded as a specific phase of the carrier signal, e.g. 0 degree or 180 degrees. The amplitude spectrum of the BPSK signal is a sinc function (i.e., sin(x)/x) whose 3 dB bandwidth is 1/T Hz, where T is the duration of each symbol.
To increase the number of bits transmitted through a fixed bandwidth, more complex modulation schemes are used to increase the number of bits represented by a symbol, so that more than one bit is transmitted during each symbol interval T. One example of such schemes is the quadrature PSK (QPSK) modulation, under which each symbol encodes two bits of information. The QPSK scheme uses 4 phase states (e.g. 0.degree., 90.degree., 180.degree., and 270.degree.) to represent the four possible values of two bits: 00, 01, 10, 11. The amplitude spectrum of the QPSK signal, like the BPSK signal, is a sinc function with a 3 dB bandwidth of 1/T Hz. However, under QPSK, two bits of information are transmitted for each symbol interval.
Higher order modulation schemes increase the number of bits encoded by a symbol. Typically, under such a higher order scheme, a symbol is represented by one of a large number of amplitude and phase states. For example, under a 16-QAM scheme, sixteen amplitude and phase states are used to define 16 distinct symbols. Thus, under the 16-QAM scheme, four bits of information are transmitted per symbol interval. 16-QAM occupies the same bandwidth as the BPSK signal.
Each of the digital modulation methods mentioned above uses modulation of a single carrier signal. Other methods exist in the prior art which represent a symbol of information during a single symbol interval using one or more carrier signals. One class of schemes using more than one carrier signal is referred to as "m-ary orthogonal signaling". M-ary orthogonal signaling uses orthogonal waveforms to represent information symbols. One example is the binary frequency shift-key ("binary FSK") scheme, under which one of two orthogonal frequency states is transmitted per symbol interval. One frequency state, represented by one of the two orthogonal frequencies, encodes a bit value of `0`; the other frequency state, represented by the other of the two orthogonal frequencies, encodes a bit value of `1`. At the receiver, a matched filter is provided for each of the orthogonal waveforms to determine which bit value was transmitted during a particular symbol interval. M-ary orthogonal signaling can transmit more than one bit during a symbol interval by assigning a bit pattern to each othogonal waveform. The receiver then uses a parallel bank of matched filters to recover which of the orthogonal waveform was transmitted. Under m-ary orthogonal signalling, each matched filter detects the energy of one of the orthogonal waveforms. During any given symbol interval, since only one of the orthogonal waveforms is transmitted, only one of the matched filters should detect significant signal energy.
Under a scheme for simultaneous transmission of orthogonal sinusoids, N time-limited filtered complex sinusoids are transmitted in parallel, where N is an integer greater than or equal to two. The N complex sinusoidal frequencies are spaced by the symbol rate (i.e., 1/T, where T is the symbol interval). Because the sinusoid frequencies are weighted by a finite impulse response filter, the resulting transmission channel consists of N spectrally overlapping sub-channels or sub-bands. The amounts of spectral overlap are determined by the spectral shape of the filter.
Examples of simultaneous transmission of orthogonal sinusoids include (i) "An Orthogonal Multiplexed QAM System Using the Discrete Fourier Transform" by B. Hirosaki, IEEE Transactions on Communications, Vol. Com-29, No. 7, July 1981, pp.982-89; (ii) "Advanced Groupband Data Modem Using Orthogonal Multiplexed QAM Technique" by B. Hirosaki et al, IEEE Transactions on Communications, Vol. Com-34, No. 6, June 1986, pp. 587-92; and (iii) "Synthesis of Band-Limited Orthogonal Signals for Multichannel Data Transmission," by R. Chang, Bell System Technical Journal, December 1966, pp. 1775-96.
One disadvantage of these schemes for simultaneous transmission of orthogonal sinusoids is the interchannel interference resulting from the overlapping spectra. Interchannel interference can be mitigated by using a filter having a narrower spectrum, or equivalently, a filter having a longer duration on each orthogonal sinusoid. However, a longer duration filter requires more computation and spans more symbol intervals, thereby resulting in the possibility of intersymbol interference. Finally, methods for simultaneous transmission of orthogonal sinusoids are often designed to be multiplexing schemes for combining data from multiple data streams for transmission in a single channel. The filtering required under such methods often involves multiple symbols, thereby complicating the application of these methods to a single stream of data.
Hybrid Frequency and Phase Shift Keying (FPSK) is another scheme for simultaneous transmission of orthogonal sinusoids. Unlike the other schemes for simultaneous transmission of orthogonal sinusoids discussed above, FPSK multiplexes a single data stream. Under FPSK, each complex sinusoidal basis function is modulated by conventional PSK, and the number of basis functions is typically chosen to be two or four. One implementation of FPSK modulation is described in "On the Performance of a Hybrid Frequency and Phase Shift Keying Modulation Technique", by Khalona et al, IEEE Transactions of Communications, Vol. 41, No. 5, May 1993, pp. 655-659. The complex sinusoids in Khalona's FPSK scheme are not individually filtered, as is done in the other methods for simultaneous transmission of orthogonal sinusoids described above. However, filtering the composite signal is consistent with the methods for simultaneous transmission of orthogonal sinusoids.
Several techniques have been applied at the transmitter to improve the spectral efficiency of digitally modulated signals. One technique applies a bandpass filter to a modulated signal prior to transmission to minimize frequency sidelobes. Another technique, called "partial response", passes the symbols through bandpass filters to achieve spectral shaping and to introduce spectral nulls.
Yet another technique, known as "continuous phase modulation", is applicable to FSK formats. In continuous phase modulation, each symbol is represented by a sinusoid selected such that an integral number of cycles of that sinusoid fits into the symbol duration. Consequently, by starting each symbol at the same phase, phase discontinuities at the symbol boundary are avoided. Avoiding phase discontinuity is desirable, since a phase discontinity creates high frequency components in the modulated signal. When filtering is applied to such a modulated signal to obtain a band-limited signal, the attenuation of the high frequency components necessarily creates distortion in the amplitude or envelope of the filtered signal. Minimum shift keying (MSK) is a special case of continuous phase modulation of FSK.
Both the transmission of a digitally modulated signal through a band-limited communication channel (i.e., a kind of bandpass filtering) and the use of the partial response technique mentioned above result in intersymbol interference. The form of interference resulting from the partial response technique is known as "controlled" intersymbol interference. To compensate for intersymbol interference, including controlled intersymbol interference, a channel equalizer can be provided at the receiver.
Demodulation of conventional digital modulation formats includes the steps of (i) symbol synchronization to enable discrete sampling of the input waveform at symbol intervals, and (ii) carrier synchronization to down-convert the sampled signal to a baseband signal. Thereafter, the resulting baseband signal samples are equalized to compensate for channel distortion and interchannel interference. The equalizer is typically an adaptive digital filter having automatically adjustable filter weights. The equalizer can also be part of a decision-aided carrier tracking and down-conversion loop which centers the signal spectrum at 0 Hz. A decision processor is applied at the output of the equalizer to recover the information symbol from the equalized signal. In higher order modulation formats, the symbol is then converted to the bit or bits the symbol encodes.